Environmental variables influencing the incidence of Pierce's disease
Background and Aims: Pierce's disease (PD) of cultivated grape, Vitis vinifera, caused by the bacterium Xylella fastidiosa (Xf) and transmitted by xylem-feeding insects, can lead to vine decline and death. Infection and expression of PD among or within vineyards vary, depending on known environmental variables, such as climate and Xf vegetative reservoirs, as well as some which are little studied with respect to PD, such as soil, water and nutrition. Methods and Results: We collected data on over 30 environmental variables and used canonical correspondence analysis (CCA) and discriminant analysis (DA) to analyse their relationship to PD infection, sharpshooter density and Xf reservoir plants. The CCA found high positive correlation between PD and vineyard proximity to citrus orchards, and moderately positive correlation between PD and grape petiole Zn concentration and soil pH. The DA retained eight variables and predicted an increased risk of PD with higher cover of annual grasses, proximity to riparian habitat, increasing soil pH and increasing petiole Zn; PD risk decreased with higher perennial grass cover, higher latitude, increasing vine water stress and increasing soil cation exchange capacity. Conclusions: We identified several novel variables which contributed to expression of PD and may improve the understanding of the role of environment in the disease triangle. Significance of the Study: That the models include factors relating to soil and soil/plant relations suggest that these variables and their interactions play a role in constitutive or induced plant defenses to Xf and provides a basis for further study into management options for PD.